How to Rotate and Animate object every other angle?
Driver Approach

You can type a driver directly into field using #.  Typing 

#radians(137.5 * frame)


into the rotation.z field of an object will make it spin at 550rpm at 24 fps.

In the added file I've set up two custom properties "spin", how many degrees per frame, and "rpm". 



The rotor rotates

radians(spin * frame)


and the rpm is calculated via a driver by

rpm = fps * spin * 60/360




Original post: https://blenderartists.org/forum/showthread.php?229688-Helicopter-Rotor&amp;p=1937512&amp;viewfull=1#post1937512